1. Field of the Invention
The present invention relates to an ink jet printing apparatus and control method thereof. In particular, the present invention relates to a method for correcting printing position displacement for an ink jet printing apparatus wherein a plurality of ejection port arrays are aligned each being extended in the widthwise direction of continuous printing paper such as rolled paper, and the printing of an image is performed on the continuous printing paper.
2. Description of the Related Art
In a color ink jet printing apparatus which uses a full-line head, a plurality of ejection port arrays which are used to eject inks in different colors are arranged at predetermined intervals in a direction in which a printing medium is to be conveyed. Therefore, when print dots are formed at one position of the printing medium, timings for ejecting ink must be shifted for the individual ejection port arrays. In order to adjust the ink ejection timings, there is a well known method that null data which represents no ejection of ink is added to print data which is to be printed the individual ejection port arrays, while the amount of null data to be added differs, depending on the ejection port arrays.
Generally, the null data is set by a unit of predetermined bits so as to be easily processed by a CPU. Accordingly, the intervals of the ejection port arrays are also set as enable the adjustment of the ink ejection timings, and thus, it is difficult to set the interval of ejection port arrays arbitrarily.
In Japanese Patent Laid-Open No. 2004-330771, in order to set an arbitrary interval of the individual ejection port arrays, a method that null data which differs in volume is added to the individual ejection port arrays, and a start address for reading the null data is changed in accordance with the positioning of the ejection port arrays is disclosed. According to the method disclosed in Japanese Patent Laid-Open No. 2004-330771, when the null data that is added is a multiple of the unit of predetermined bits, the reading of the null data can still be started at a proper address in the middle of the unit of bits, and therefore, a plurality of ejection port arrays can be set at arbitrary intervals.
Incidentally, the ink jet printing apparatus has conveying means (a conveying mechanism) for conveying printing media. There are cases that change have occurred on the surfaces of conveying rollers which are used as conveying means, and the change have caused the distances in which printing media were conveyed to fluctuate, and as a result, the printing positions were shifted. Furthermore, the conveyance amounts of printing media are changed by the moisture content of the printing media themselves.
According to the arrangement described in Japanese Patent Laid-Open No. 2004-330771, the positions of the ejection port arrays are used as reference positions when setting fixed start addresses for reading print data for the individual ejection port arrays. Therefore, in a case that the distance that a printing medium is conveyed is altered as a consequence of the condition of either the conveying means or of the printing medium, the ink ejection timing can not be appropriately adjusted. There is another adjustment method whereby the ink ejection timing can be changed during the printing of a print medium, but when this method is employed to adjust the printing start positions of the individual ejection port arrays, the transfer of ejection port drive data need to be temporarily halted. Accordingly, the printing operation also needs to be halted, and as a result, a great deal of time is required to complete the printing.